Move ?@attr, !@attr to builtins

- Each attr becomes a separate builtin, just like before with @attr.
Use ?AT_*, !AT_*, @AT_* instead of ?@*, !@*, @*

- For consistency with ?TAG_*, !TAG_*, though the namespacing argument
  is not as strong with attributes, due to them being always prefixed
  with @.  But now we can easily distinguish DW_AT_* sort of
  attributes from other kinds--such as fields of individual
  @AT_stmt_list elements.
Move ?TAG_*, !TAG_* to builtins